OK, Fabris. I made a quick javascript program to do what you're looking for. It's .hta file to overide the browser's security limitations that won't allow you to modify document.whatever in one window from another window, so it will probably ask you to save the file or run it from it's location.

First of all it has 1 limitation: The login form has to be the first form on the login page, which it is on most site's login pages.

Other than that, just follow the directions and it should work. Feel free to modify it and fix it up.

By the way, I guess I should mention that I only tested this IE 5.5.